1. Field of the Invention
The present invention relates to a DC coupling type optical receiver to be used in a data transmitting apparatus for such as a local area network.
2. Description of the Related Art
Generally, in an optical local area network comprising a star network, there is used the so-called burst data transmission in which data is not transmitted when unnecessary and transmitted when necessary in order to efficiently transmit data. The burst data transmission is characterized in that a level of a receiving signal abruptly varies for a short time unlike the general continuous data transmission between two points. In other words, in a case case where a signal having a high level arrives after a long silence period or a case where a signal sent from a transmitting station B, which is located far away a receiving station, arrives right after a signal sent from a transmitting station A, which is located close to the receiving station, the receiving level largely varies by the difference between the station A and the station B in the loss of transmission line.
Such an optical receiver to be used for receiving burst data must be satisfied the following three requirements:
First, time for which the signal can not be received is made extremely short in the case of the abrupt variation of the receiving level.
Second, a receiving dynamic range is wide.
Third, the optical receiver must be stably operated under even environmental variations such as a temperature variation, a power voltage variation, and the like.
In conventional, as a method for satisfying the first requirement, the optical receiver is formed by a DC coupling amplification. According to a method using an AC coupling amplification, which is employed well in general, it takes relatively much time to converge a discrimination level to an average value after the receiving level varies. Due to this, it is difficult to correctly discriminate data for the period of time. In contrast, according to the method using the DC coupling amplification, "0" level of a pulse signal, serving as a reference of the variation of the receiving level, is transmitted to a discrimination level, so that a quick response can be performed.
However, in the method using the DC coupling amplification, influence of drift on an amplifier must be considered. As the optical receiver in which influence of drift on the amplifier is removed, for example, Published Examined Japanese Patent Application 3-7175 discloses a method using a dummy circuit having the same circuit constant. The disclosed optical receiver uses a method of a fixed threshold value in which an output of the dummy amplification circuit is partially pressurized and a discrimination threshold value of data is set. Due to this, there is a problem in that the variation of the pulse width of discriminated data increases in accordance with the variation of the receiving level.
Moreover, in addition to the static variation such as a temperature drift, the amplifier has a dynamic variation characteristic in which the operation point varies depending on the value of the receiving level. The dynamic variation depending on the value of the receiving level cannot be detected by the dummy circuit. Due to this, it is required that the discrimination threshold value of data be set higher in consideration of the dynamic variation. However, if the threshold value is set to be higher, there occurs new problems in which the minimum receiving level is deteriorated and a wide receiving dynamic range cannot be obtained.
In order to solve the above problems, for example, Published Unexamined Japanese Patent Application No. 62-154928 discloses a method for varying the threshold value in accordance with the change of the receiving level. According to this method, a peak value of the receiving signal is detected and the discrimination threshold value is varied. Also, the drift of the amplifier is removed by the dummy circuit. Other than the above method, there is a method in which the change of the discrimination threshold value and the removal of drift are carried out by a positive and negative peak detector.
FIG. 1 is a block diagram showing the structure of the DC coupling type optical receiver using the positive and negative peak detector. In this drawing, reference numeral 100 is a photodiode for converting an optical signal to an electric signal. An output current of the photodiode of the photodiode 100 is supplied to an amplifier 200. The amplifier 200 amplifies the output current of the photodiode 100 and converts the output current to a voltage signal. The voltage signal is supplied to first and second peak detectors 400 and 800, and an input terminal (+) of a comparator 600. The first peak detector 400 detects the peak value of "0" level side of data from an input signal. The second peak detector 800 detects the peak value of "1" level side of data from an input signal. Each detection voltage is synthesized via partial pressure resistors RD1 and RD2, and supplied to an offset voltage adder 500.
The offset voltage adder 500 adds an offset voltage set in advance into an input voltage, so that the output of the comparator 600 for a silent signal input time is fixed to "0." The added voltage is supplied to an input terminal (-) of the comparator 600 as a compare reference voltage. The comparator 600 compares the voltage signal, which is sent from the amplifier 200 and supplied to the input terminal (+), with the compare reference voltage to be sent to the input terminal (-), and discriminates between receiving data "0" and receiving data "1."
FIG. 2 shows an operation waveform of FIG. 1. In FIG. 2, (f) is an output voltage waveform of the amplifier 200, (g): a waveform of the compare reference voltage; and (h) is a waveform of the output voltage of the comparator 600. VGA is an offset voltage level.
It is assumed that a signal sent from a transmitting station B, which is located far away a receiving station, arrives right after a signal sent from a transmitting station A, which is located close to the receiving station, and the receiving level largely varies by the difference between the station A and the station B in the loss of transmission line. The second peak detector 800 detects the peak value of "1" level side of receiving data. Due to this, if the receiving level changes the peak voltage can be obtained in accordance with the change of the receiving level.
On the other hand, the first peak detector 400 detects the peak value of "0" level side of receiving data. Due to this, the voltage corresponding to the DC operation point of the amplifier 200 is detected and output regardless of the change of the receiving level. Therefore, a voltage corresponding to an amplitude value of receiving data is generated between the output of the first peak detector 400 and that of the second peak detector 800. Then, a signal, which the above generated voltage is divided by resistors RD1 and RD2, and the offset voltage VGA are added, and the added result is used as the compare reference voltage (g).
As shown in the drawing, the compare reference voltage (g) changes in accordance with the size of of the receiving level, and the discrimination is performed at substantially 1/2 level of amplitude level of receiving data by the comparator 600 in a case where resistors RD1 and RD2 are equal to each other. The variation (drift) of the operating point of the amplifier 200 due to the temperature change is detected by the peak detector 400, and the amplifier 200 operates such that the compare reference voltage is varied in the same direction by an amount, which is equivalent to drift. The comparator 600 amplifies the difference between the positive and negative inputs and operates to remove the change of in-phase voltage. Due to this, influence of drift is removed and the stable discrimination can be performed.
The conventional optical receiver disclosed in Published Unexamined Japanese Patent Application No. 62-154928 or shown by FIG. 1 can realize a good burst receiving characteristic by drift compensation and control of the discrimination threshold value. However, there is a disadvantage in that the circuit structure becomes complicated, that is, a threshold voltage selection circuit having a relatively complicated structure is needed or positive and negative peak detectors are required.